By modern electronics, useful information (for example voice, measurement data, music, control commands, etc.) is transmitted, processed or otherwise manipulated by signals in analog (A) form or digital (D) form. Signal conversion between analog (A) and digital (D) signals in both directions is thereby often required. The quality of analog-to-digital (A/D) converters and digital-to-analog (D/A) converters and other electronic circuits can be expressed by, for example, a signal-to-noise ratio (SNR) indicating how useful signals are separated from unwanted signals.
Sigma-delta modulators serving as analog-to digital converters are well known in the art and had been described in a variety of publications. For the application of sigma-delta modulators and for prior art designs, the following references are useful:
1! Franca, J. E., Tsividis, Y. (editors): "Design of Analog-Digital VLSI Circuits for Telecommunications and Signal Processing", Second Edition, Prentice Hall, Englewood Cliffs, 1994, ISBN 0-13-203639-8, chapter 10 "Delta-Sigma Data Converters" by Temes, G. C., on pages 317-339; PA0 2! Norsworthy, S. R., Schreier, R., Temes, G. C.: "Delta-Sigma Data Converters", IEEE Press, New York, 1997, ISBN 0-7803-1045-4; and PA0 3! Proakis, J. G., Manolakis, D. G.: "Digital Signal Processing", Third Edition, Prentice Hall, Englewood Cliffs, 1996, ISBN 0-13-373-762-4, chapter 9.2. "Analog-to-Digital Conversion", on pages 748-762, and chapter 3.1. "Z-Transform", on pages 151-160.
FIG. 1 illustrates a simplified block diagram of sigma-delta modulator 20 (hereinafter modulator 20) as known in the prior art. Modulator 20 comprises adder 22 ("+"), integrator 24 ("INT"), comparator 26 ("CP"), and digital-to-analog (D/A) converter 28. Optionally, modulator 20 also comprises dither generator 30 (U-shaped dashed frame) having, for example, digital dither generator 32 ("DITHER", producing signal d'), digital-to-analog (D/A) converter 34 (producing signal d), adder 36 ("+") and subtractor 38 ("-").
Elements within dashed frame 20' are considered first with signal d assumed to be initially set to zero, i.e. signal d=0. Adder 22 receives analog input signal x and feedback signal -z and provides (x-z) to integrator 24. Integrator 24 integrates (x-z) and provides .intg.(x-z) to comparator 26. Comparator 26 compares .intg.(x-z) to a reference signal r and provides output signal y as a series of binary values such as, e.g., "+1" and "-1". The sequence of the "+1"and the "-1" values in y represents the analog input signal x. D/A converter 28 uses output signal y to provide the feedback signal -z to adder 22.
The digital output signal y of modulator 20' (within frame 20') can show unwanted signals, such as for example, periodical fluctuations. This happens, for example, when analog input signal x is constant over a certain time period, or when the input signal x is small in comparison to its signal range. Adding dither signals d, for example, to input signal x is used in the art to improve the performance of the modulator. In dither generator 30, digital dither generator 32 provides a digital random signal d' and D/A converter 34 transforms d' to an analog dither signal d. Adder 36 adds dither signal d to analog input signal x and subtractor 38 subtracts d' from digital output signal y.
However, such conventional dithering suffers the following main disadvantages: (a) Dither signal d goes through modulator 20 and increases the overall noise within output signal y: (b) Dithering further limits the dynamic range of x (e.g., ratio between .vertline.x.sub.max .vertline. and .vertline.x.sub.min .vertline.) because elements which receive x (e.g., adder 22), must not only accommodate, e.g., .vertline.x.sub.max .vertline., but also must accommodate, e.g., .vertline.x.sub.max .vertline.+.vertline.d.sub.max .vertline..
FIG. 2 illustrates a simplified amplitude-frequency diagram 10 of output signal y of sigma-delta modulator 20. Diagram 10 gives an amplitude .vertline.y.vertline.(.vertline. .vertline. for absolute values) of signal y on the vertical axis versus increasing frequency f on a horizontal axis. Such a presentation is well known by the term "spectrum". A person of skill in the art is able to measure such spectra without the need for further explanation. Trace 11 shows the spectrum of signal y' of modulator 20 having no dither generator 30 (dashed frame 20' in FIG. 1). Signal y' also has spectral lines 13 for certain frequencies f.sub.1, f.sub.2, etc., also called "spectral tones". The tones are not wanted. For example, they can be amplified by circuits coupled to the output of modulator 20 or they can cause incorrect operation in these circuits. Trace 12 shows the spectrum of signal y of modulator 20 employing dither generator 30. Dither signal d is preferably, "noise-shaped " with, for example, amplitudes .vertline.d.vertline. related to the frequency f proportionally or by a square function: EQU .vertline.d.vertline..about.f (1) EQU .vertline.d.vertline..about.f.sup.2 ( 2)
Spectral tones in y' are substantially reduced in signal y. However, dithering introduces noise and the amplitude of y is higher than the amplitude of y'. In other words, the SNR of modulator 20 (with dither) is lower than of modulator 20' (without dither).
There is an ongoing need to provide an improved analog-to-digital converter which mitigates these and other disadvantages and limitations of the prior art.